Systems for measuring the moisture content of sheet material (such as paper) during its production are well-known in the art. A class of such systems makes use of the selective absorption of certain wavelengths of infrared radiation by water.
In the typical operation of these systems, infrared radiation is directed toward the sheet from a source within a traversing sensor. The radiation interacts with the sheet via absorption, scattering, or transmission. By suitable optical filtering, unabsorbed radiation of at least two different wavelengths is separately detected by one or more detectors, which in turn produce two or more voltage responses from which a measurement of moisture content is derived. The following patents exemplify systems of the type generally described above: U.S. Pat. Nos. 3,228,282 Barker, and 3,405,268 Brunton.
A number of modifications to the above-described systems have been made to compensate for measurement error resulting from changes in composition of the sheet material, or from variations in scattering power. Thus, U.S. Pat. No. 4,052,615 Cho discloses an apparatus with two hemispherically concave reflecting surfaces having a particular dimension in relation to the pass gap (distance between a source unit and a detector unit) and to the diameter of the source beam in order to compensate for variations in the scattering characteristics of the sheet. U.S. Pat. No. 4,306,151 Chase discloses a method for measuring the moisture content of paper that is contaminated with carbon. U.S. Pat. No. 4,577,104 Sturm discloses apparatus for measuring the moisture content of sheet material having both variable scattering characteristics and a contaminant, via separate measurement of absorption at four different infrared wavelengths.
Other complications in measuring the moisture content of sheet material, such as paper, derive not so much from the presence of contaminants or variations in scattering power as from the relative amounts of usual constituents of the material which are present in a particular application. U.S. Pat. No. 3,793,524 Howarth discloses an apparatus for measuring moisture content in light-weight papers such as tissues, whereby an offset geometry is employed in conjunction with two planar reflecting surfaces to effect multiple interaction of infrared radiation with the sheet material in order to increase the sensitivity of the apparatus to moisture content.
Another complication relates to temporary moisture stratification along the thickness dimension of the sheet. This is commonly caused by various process control operations. For example, it is a common practice in papermaking to alter the moisture content of the paper web at selected cross-machine zones thereof in order to control its moisture profile. This may be accomplished by a variety of methods including the use of spray dampening ("rewetting") systems or heating systems. See, e.g., U.S. Pat. Nos. 4,188,731 Rauskolb and 4,378,639 Walker.
When some of the these operations are performed, particularly that of rewetting, a temporary period of substantial moisture stratification occurs. In an absorbent material such as paper the moisture will eventually equilibrate so that moisture content is substantially uniform throughout the thickness of the sheet. However, if the moisture measurement is made during the period of moisture stratification by an infrared moisture sensor that is calibrated with samples which have uniform moisture content (which, ordinarily, must be the case), the measurement will be lower than the actual moisture content. This is believed to result from a low effective path length for the radiation as it travels through a high-moisture stratum of the sheet. The low effective path length in turn results from the relative scarcity of cellulosic fibers in the high-moisture stratum, and the correspondingly low amount of scattering in that layer.
The above moisture control operations are typically not performed to a consistent degree for a given cross-machine zone of the sheet material. Nor are they typically performed to the same degree in all cross-machine zones. For example, a spray dampening system may rewet the sheet in a particular zone for a given period of time at a given rate of application, and then at a different rate of application during a subsequent period, or it may cease application altogether. Therefore, there is a need for an infrared moisture sensor which will accurately measure the moisture content of sheet material under conditions of either a uniform or a stratified moisture distribution.
It has been discovered that by adjusting the relative, effective reflectivities of two areas of the sensor which generally face opposite surfaces of the sheet material, an accurate measurement can be produced under both of the above-stated conditions.